1. Field of the Invention
This invention relates to extraction of contaminants from soil and is directed more particularly to in situ extraction of contaminants from unsaturated soil.
2. Description of the Prior Art
Heavy-metal contamination of soil and groundwater is a widespread problem in industrial nations. Large spills and leaks can contaminate both the soil above the water table and the aquifier itself. Smaller spills, or spills in arid regions with thick vadose zones, can result in a contaminant plume that totally resides in unsaturated soil. In both cases, there exists a contamination problem in the vadose zone in need of remediation. Excavation of such sites often are not cost effective or politically acceptable. Electrokinetic remediation is a technique utilized for in situ removal of such contaminants from soil.
To conduct electrokinetic remediation, electrodes are implanted in the ground and a direct current is imposed between the electrodes. The application of direct current leads to two effects; ionic species in the soil-water solution migrate (electromigration) to the oppositely charged electrode, and accompanying this migration, a bulk flow of soil-water is induced toward one of the anode and the cathode by electroosmosis, usually toward the cathode. The combination of these two phenomena leads to a movement of contaminant ions towards one or the other, or both, of the electrodes.
The direction and rate of contaminant movement is determined by a number of factors, among which are type and concentration of contaminant, soil type and structure, interfacial chemistry of the soil-water system, and the current density in the soil pore water. Contaminants arriving at the electrodes may potentially be removed from the soil by one of several known methods, including pumping out of the soil water having the contaminants therein.
The direction of water movement in the soil is determined by the electrical charge of the soil. In most instances, soil is negatively charged and water will move therethrough to the cathode. However, under some conditions, some soils may have a positive charge and the water therein will, upon actuation of an electric field, flow toward the anode.
The above method is suited to operate in a saturated zone. Pumping water from a well that contains an electrode appears to be the most common electrokinetic extraction method. However, unlike groundwater in saturated soil, pore water in a vadose zone is held under tension in the soil pores. This tension prevents the pore water in the vadose zone from flowing into extraction wells as it does in the saturated zone. Therefore, effluent extraction techniques at an electrode, proposed for saturated methods, are not applicable in the vadose zone.
In U.S. Pat. No. 5,074,986, issued Dec. 24, 1991 to Ronald F. Probstein, et al., there is disclosed a method for removing contaminant material from a soil site. In the Probstein process, electrodes are placed in the ground and an electrical potential is imposed between the electrodes, which induces an electrical current. A purge solution is introduced under pressure at the anode and electroosmotically passed through the soil, which displaces contamination into the cathode well. However, in unsaturated soils the purge solution is drawn into the soil at a rapid rate and can drain from the site before reaching the cathode well, carrying contaminants to new depths.
In U.S. Pat. No. 5,137,608, issued Aug. 11, 1992 to Yalcin B. Acar, et al., there is disclosed a process similar to the Probstein process and stated to be operative in saturated soils. In Acar, water is added to the soil at the anode and an acid front is produced by reactions at the anode, the acid front moving through the soil causing contaminant ions to be transported to the cathode. Again, the use of the Acar method in unsaturated soils could result in drainage of water, with consequent transportation of contaminants deeper into the ground.
While the procedures taught by Probstein et al., ,and Acar, et al., provide means and methods for removing contaminants from saturated soil, there remains a need for an electrokinetic remediation device, system, and method by which unsaturated soils can be cleansed of contaminants in situ without having to saturate the soil, which saturation carries concomitant risk of washing the contaminants deeper in the soil and/or outside of the treatment zone.